This invention relates to the reproduction of video recordings, and more particularly to improvements in video tape players of the type which employs a multichannel, essentially-fixed, magnetic head to reproduce a sampled-analog video signal recorded in a plurality of parallel tracks on a video tape. The term "essentially-fixed" as used herein with respect to magnetic heads is intended to distinguish linear recording apparatus from helical-scan and similar type apparatus wherein the recording head(s) moves with respect to the apparatus during recording or playback.
An example of an essentially-fixed head video recording apparatus for recording a sampled-analog video signal on a plurality of tracks, and the advantages of such apparatus over the prior art, are disclosed in copending U.S. patent application Ser. No. 957,269 entitled MULTICHANNEL RECORDING FORMAT FOR A SAMPLED-ANALOG COLOR VIDEO SIGNAL by J. R. Horak et al and filed on the same day as the present application.
The above-referenced copending application discloses a video camera/recorder having a multichannel sampled-analog recording format wherein a color video signal representing successive groups of analog color samples, generated, for example, by a solid state image sensing array(s), is assigned, in a novel manner, to a plurality of channels. The signal is separated, sample-by-sample, and such samples are assigned to respective channels of a plurality of channels such that the respective channels contain subgroups of samples representing only one color per subgroup. The signal in the channels is recorded on a magnetic tape in a plurality of parallel tracks, one track per channel, by an essentially-fixed, multichannel recording head.
When such a signal is being reproduced from the recording, an essentially-fixed multichannel playback head reproduces the signal portion appearing in each track and applies such signal portion to a corresponding channel. Signal processing apparatus similar to that employed to separate the signal on a sample-by-sample basis is used to recombine the samples from the respective channels to produce an output signal which may have the same form as the original signal. Ideally, to effectively produce the output signal, the tape is moved past the playback head at the same speed and with the same relative orientation between the head and the tape that were employed during recording; in addition, the relative alignment among the gaps in the playback head must match exactly that of the recording head.
In reality, the above ideal conditions are difficult to achieve, and as a result, several forms of time base error distortion may be present in the reproduced signal. Such distortion may result in a severely degraded or totally incomprehensible video display.
One form of time base error--called flutter--results from variations in the speed of the tape moving past the record or playback head. These variations may be caused by localized tape stretch, tape vibration, and/or fluctuations in the speed of the tape transport mechanism. Flutter in the reproduced video signal causes time-base instability in the synchronizing information within the signal, which may degrade the quality of the video display.
A special case of flutter--called drift--may result from an overall tape stretch, and/or a slight mismatch between the tape transport speeds during recording and playback. Although drift, which may be thought of as zero-frequency, high-amplitude flutter, may not adversely affect the display of the reproduced video signal, it must be taken into account when recombining the signal portions carried by respective channels during reproduction of the original signal.
Another form of time base error that may prohibit effective recombination of the signal portions recorded in respective channels is called skew. Skew results from angular misalignment between the tape and the playback head caused by mechanical misalignment of the head in the apparatus and/or by an angular displacement of the tape during transport past the head. The tape may, in effect, squirm past the head during recording or playback, thereby introducing varying amounts of skew in the signals recorded thereon. Skew causes the signal portions in respective channels to be out of synchronization relative to each other and therefore interferes with effective recombination of the signal portions to reproduce the original video signal.
A third form of time base error, called scatter, results from variation in the relative positions of the gaps of the record head with respect to the relative positions of the corresponding gaps in the playback head. The effects of scatter on the effective recombination of the signal portions recorded in the multiple channels are similar to those of skew.
Thus, it is desirable to provide means for removing signal distortions such as flutter, scatter, and skew from the video signal reproduced by an essentially-fixed head from a multitrack video recording.
One prior art system for removing skew from a multichannel video signal employs a voltage variable analog delay line in each channel. See U.S. Pat. No. 3,327,299 issued June 20, 1967 to W. R. Johnson. Periodic reference signals such as sync pulses recorded in each channel are employed as feedback signals to develop control signals for the delay lines. Although skew is thus removed between signal portions in the various channels, time base instability of the overall signal resulting from flutter is not.
Another system employing analog delay lines, see U.S. Pat. No. Re. 25,809 issued June 22, 1965 to W. R. Johnson, addresses the problem of removing flutter and skew from a multichannel video signal. One problem with prior art devices employing voltage variable analog delay lines lies with the delay lines themselves. Such delay lines tend to delay different frequencies by different amounts, thereby introducing phase distortion into the delayed signal.
Some recently developed circuits for correcting the time base of single channel video signals employ charge transfer type analog shift registers as delay elements. One such circuit, employing a bucket brigade type device as an analog delay line, is shown in U.S. Pat. No. 3,959,815 issued May 25, 1976 to Rotter et al. A video signal, to be defluttered, is passed through the bucket brigade device. Upon exiting the device, the horizontal sync pulses are stripped from the video signal and employed as a feedback signal in a phase-locked-loop clock generator which is also supplied with a fixed frequency reference signal. The clock signal thus generated controls the operation of the bucket brigade to either slow down or speed up the transition of the video signal through the bucket brigade device. Such an approach, although possibly adequate for removing relatively low amplitude high frequency flutter from a single channel video signal, is severely limited when the signal is a multichannel, sampled analog video signal exhibiting combined flutter, scatter and skew that may approach channel-to-channel time base differences of one line scan, i.e. approximately 60 microseconds. A bucket brigade device employing feedback control capable of providing compensation for delays of such magnitude, while preserving the information content of the sampled analog video signal, would have to provide a very long delay indeed, requiring a prohibitively large number of elements.